Toralei complicates the Ghoul Squad’s latest mission to find a hidden monster deep inside some ancient catacombs when she tries to take an artefact that triggers an avalanche of the ancient cave’s traps and defences.
The Ghoul Squad are helping Frankie and Venus find some music for their science project, where they plan to test if monster plants can actually respond to music. While searching, Clawdeen discovers a story about a musical ghost being sighted in a set of caverns. Suspecting it may be a new monster to rescue, the ghouls head off, while Frankie and Venus continue with their project.
The ghouls head to the library, where they run into Toralei. She laughs at the ghouls and says they shouldn't bother looking for more monsters now that she's joined the school. When they ask why she's in the library, she admits Dracula put her on duty for letting a zombie hamster loose in the creepateria.
Dracula discovers some rumours of humans hearing mysterious music inside some lost ruins, and suggests that the ghouls follow the music in order to find their monster. As they prepare to leave, Toralei plays about on the computer, and spots an interesting article...
Just as the ghouls are ready to leave, Toralei begs them to let her come with them. The ghouls are reluctant, but Draculaura suggests that bringing her along might do her some good. The group teleports to the entrance of a cave system. Clawdeen's ears hear a distant sound, and they head deeper in.
Meanwhile, Frankie and Venus are trying to stimulate a monster plant with music, but the plant fails to respond. Frankie accidentally plays some heavy metal music, and the plant suddenly comes to life, and starts headbanging to the beat. Frankie tries to play some other music, but the plant snatches her phone away.
The ghouls search through the underground tunnels, while Toralei tries to sneak off on her own, to no success. At a dead end, the floor crumbles and the ghouls fall into a minecart, which races down a track right into a deep drop. They narrowly avoid plummeting to their doom. They hear the music again, and discover a large cavern, where a phantom ghoul is playing a guitar.
The ghoul introduces herself as Operetta. She explains that she played her music in order to scare off humans away from the cavern, as she loves the accostics in it... and because the cavern floor is covered in diamonds. She explains the cavern was once used by an ancient civilisation to store their treasure. She invites the ghouls to help themselves to some diamonds, but warns them not to take the Great diamond in the center of the room... which Toralei immediately steals. The ghouls realise this was the reason she joined them in the first place.
The cavern begins to collapse and cave-in. The ghouls try to teleport away, but the Mapalogue is unable to work deep underground. Operetta leads the group through some tunnels. They narrowly avoid some pitfalls and cave-ins, but manage to escape the caves. Toralei is disappointed when she realises she failed to make off with any off the diamonds. Operetta accepts the ghoul's offer to come to Monster High.
The group return to the school, where they discover loud rock music blaring, and the now giant-sized monster plant tossing Frankie and Venus around. Operetta plays her guitar to distract the plant, allowing Frankie to switch the music off. Unfortunately, Operetta finds she's stuck entertaining the plant for a while...
Operetta makes her first animated appearance in Generation 2.